1. Field of the Invention
This invention relates to topical and transdermal drug formulations. In another aspect this invention relates to formulations containing prostaglandins or derivatives thereof, particularly prostaglandin E.sub.1 esters.
2. Description of the Related Art
Topical and transdermal drug formulations are designed to deliver a therapeutically effective amount of drug to or across the skin of a patient. Devices known to the art include reservoir type devices involving membranes that control the rate of drug release to the skin, gels and creams, and devices involving a dispersion of the drug in a matrix such as a pressure sensitive adhesive. As the skin presents a barrier to the drug it is often desirable or necessary to incorporate certain materials that enhance the rate at which the drug passes through the skin. For any particular drug, however, the type of device, the transdermal flux rate that is suitable, and suitable formulation components, are dependent upon the particular drug to be delivered.
Prostaglandins as a class of compounds have diverse pharmacologic activity, including stimulation of gastrointestinal and reproductive smooth muscle, relaxation and contraction of respiratory smooth muscle, hypotensive activity, inhibition of fatty acid lipolysis, inhibition of blood platelet aggregation, and inhibition of gastric acid secretion. Therapeutic utility of prostaglandins in general is correspondingly broad. As for prostaglandin E.sub.1 ("PGE.sub.1 ")in particular, this compound, salts thereof, and lower alkyl esters thereof are well known and disclosed, e.g., in U.S. Pat. Nos. 3,069,322 (Bergstrom et al.), 5,219,885 (Froelich et al.) and in J. Org. Chem. 1974, 37, 2921. PGE.sub.1 has found utility in the treatment of peripheral occlusive diseases, acute myocardial infarction, angina pectoris, acute ischaemic stroke, asthma, gastrointestinal ulcers, ulcers of the skin, and organ rejection. Various routes of administration have been described, including oral, intravenous, buccal, rectal, intraarterial, subcutaneous, and sublingual. The preferred route of administration of PGE.sub.1 will of course be dependent on the particular intended therapeutic use.
Topical and transdermal administration of PGE.sub.1 and PGE.sub.1 derivatives have also been described, e.g., in U.S. Pat. Nos. 4,889,845 (Ritter et al.), 4,515,810 (Chow et al.), and 5,219,885 (Froelich et al.) and in Japanese Kokai 2-264725 (Morimoto et al.) and 63-135333 (Nakano et al.). In order for a transdermal formulation of PGE.sub.1 or a derivative thereof to be effective and suitable it is desirable that the formulation have a high transdermal flux rate, allowing a therapeutically effective blood level of the drug to be achieved or maintained when the formulation is applied to a relatively small area of the skin. Furthermore PGE.sub.1 readily undergoes certain reactions and rearrangements (see. e.g., J. Chromatography, 1991, 555, 73 (Lee et al.). This instability of the prostaglandin can be problematic in providing a suitable transdermal formulation.